This invention relates to a vacuum switch having high current interrupting capability and high insulating strength after current interruption.
In FIG. 1, there is shown a conventional vacuum switch which comprises a vacuum vessel defined by an insulating cylinder 1 made of an inorganic material such as ceramics or glass and end plates 2 and 2'. In the vessel there are disposed a stationary contact 4 secured to the end of a stationary rod 3 and a movable contact 6 secured to the end of a movable rod 5, and disposed to oppose the stationary contact, and a metal shield 8 is provided around the contacts to condense metal vapor generated by electric arc at the time of current interruption. A metal bellows 7 is further provided for opening and closing the contacts 4 and 6 while maintaining airtightness. Such a vacuum switch can interrupt the current by separating slightly the contacts from each other by moving the movable rod 5 outwardly of the vacuum vessel and can maintain sufficient insulating strength under the vacuum after the current interruption.
However, recently it has been required to provide a vacuum switch to be used with high voltage and large current and it has been found that such a conventional vacuum switch shown in FIG. 1 can no longer be used for electric circuits operating at a voltage higher than a certain limit because of the lowering of the insulating strength between the contacts after the current interruption, this lowering of the insulating strength being based on the fact that the surface of the contacts are roughened by the arc generated therebetween and a number of small projections are formed on the surfaces thereby extremely strengthening the local electric field causing discharge.
In certain designs, annular metal fittings are used, each of which is welded or soldered to the end plate at its one end and at the other end secured airtightly to the end portion of the insulating cylinder. In such structure, the outer diameter of the annular fitting is substantially equal to the maximum diameter of the vacuum vessel, and to this fitting is applied the same voltage as that applied to the stationary contact of a vacuum switch. Therefore, it is required to make the interphase distance of a three phase circuit breaker large and the insulating distance from the ground in the case where the vacuum switch is accomodated in a cubicle or a tank, which finally leads to the enlargement of the size of the switching apparatus as a whole.
Furthermore, after the fabrication of the vacuum vessel by means of soldering or welding, degassification of the interior of the vessel should be made through an evacuating and seal-off tube attached to one of the end plates by heating the vessel. The top end of the seal-off tube is connected to an evacuation device, and the seal-off tube is cut and sealed off at a suitable portion near the end plate after completion of the degassification. However, since the cut seal-off tube still projects from the end plate, it is obstructive for the conveyance or installation of the vacuum vessel and attention must be paid so as not to damage the tube.